Polyesters represented by polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN) etc. are excellent in mechanical and chemical characteristics, and are used in various fields for example in fibers for clothing and industrial materials, films for packaging or for magnetic tapes, sheets, hollow molded articles such as bottles, casings for electrical or electronic parts, and other molded articles such as engineering plastics, depending on the characteristics of each polyester.
As typical polyester, polyester composed of an aromatic dicarboxylic acid and an alkylene glycol as major constituent components, for example polyethylene terephthalate (PET), is industrially produced by esterification or transesterification of terephthalic acid or dimethyl terephthalate and ethylene glycol to produce bis(2-hydroxyethyl) terephthalate which is then subjected to polycondensation at high temperatures in vacuum in the presence of a catalyst.
As a conventional polyester polymerization catalyst used in polycondensation of polyester, antimony trioxide has been used widely. Antimony trioxide is an inexpensive and highly active catalyst, but when antimony trioxide is used as a major component, that is, when it is used in such an amount as to exhibit a practical rate of polymerization, an antimony metal is precipitated to cause problems such as gray discoloration or generation of insoluble particle in polyester. For this reason, polyester absolutely free of antimony or excluding antimony as a major catalytic component is desired.
The above-described insoluble particle in polyester causes the following problems.
In polyester for film, the antimony metal precipitated serves as insoluble particle in polyester, which causes not only contamination of an outlet during melt extrusion but also deficiency on the surface of film. Further, when the polyester with insoluble particle is used as a starting material of hollow molded articles, it is difficult to obtain hollow molded articles excellent in transparency.
The insoluble particle in polyester for fibers serves as insoluble particle not only causing a reduction in the strength of fibers, but also deposits around spinnerets during spinning. In production of polyester fibers, a polyester polymerization catalyst not causing formation of insoluble particle is desired from the viewpoint of productivity.
As a method of solving the problem described above, an attempt had been made at preventing gray discoloration and formation of insoluble particle in PET while using antimony trioxide as a catalyst. In addition, in Japanese Patent No. 2666502, formation of black insoluble particle in PET is prevented by using antimony trioxide, a bismuth compound and a selenium compound as a polycondensation catalyst. Further, Japanese Patent Application Laid-Open No. 9-291141 describes that precipitation of an antimony metal is prevented when antimony trioxide containing sodium and iron oxides is used as a polymerization catalyst. However, these polycondensation catalysts cannot achieve the object of reducing the content of antimony in polyester.
As a method of solving the problem of the antimony catalyst in uses requiring transparency of PET bottles etc., for example Japanese Patent Application Laid-Open No. 6-279579 discloses a method of improving transparency by prescribing the proportion of antimony and phosphorus compounds used. However, it cannot be said that hollow molded articles made of polyester obtained by this method are sufficiently transparent.
Further, Japanese Patent Application Laid-Open No. 10-36495 discloses a process for producing polyester excellent in transparency, which includes use of antimony trioxide, phosphoric acid and a sulfonic acid compound. However, polyester obtained by such a method is poor in thermal stability, and there is the problem of a high content of acetaldehyde in the resultant hollow molded article.
Polycondensation catalysts substituted for antimony type catalysts such as antimony trioxide have also been examined, and titanium compounds represented by tetraalkoxy titanate or tin compounds have previously been proposed, but there is a problem that polyester produced by using these compounds is easily thermally degraded during melt molding, and the polyester is significantly discolored.
In an attempt at solving the problem arising when such titanium compounds are used as the polycondensation catalyst, for example Japanese Patent Application Laid-Open No. 55-116722 proposes a method of simultaneously using tetraalkoxy titanate in combination with a cobalt salt and a calcium salt. Further, Japanese Patent Application Laid-Open No. 8-73581 proposes a method of using tetraalkoxy titanate in combination with a cobalt compound as the polycondensation catalyst and simultaneously using a optical brightener. By these techniques, PET discoloration occurring when tetraalkoxy titanate is used as the polycondensation catalyst can be reduced, but prevention of thermal decomposition of PET cannot be achieved.
In another attempt at preventing thermal degradation during melt molding of polyester polymerized in the presence of a titanium compound as the catalyst, for example Japanese Patent Application Laid-Open No. 10-259296 describes a method of adding a phosphorus type compound after polymerization of polyester in the presence of the titanium compound as the catalyst. However, effective mixing of the additive with the polymer after polymerization is technically difficult and leads to higher costs, so this prior art method is not practically used under the present circumstances.
A method of adding an alkali metal compound to an aluminum compound to form a polyester polymerization catalyst having a sufficient catalytic activity is also known. When such a known catalyst is used, polyester which is excellent in thermal stability can be obtained, but this catalyst using an alkali metal compound in combination should be added in a larger amount in order to attain a practical catalytic activity, and as a result, there arises at least one of the following problems attributable to the alkali metal compound in the resultant polyester polymer.
The amount of insoluble particle is increased so that when the polyester is used in fibers, the spinnability and the physical properties of fibers are getting worse, and when used in films, the physical properties of films are getting worse.
The hydrolytic stability of the resultant polyester polymer is lowered, and due to formation of insoluble particle, the transparency thereof is lowered.
There arises the phenomenon of deficiency in color tone of the resultant polyester polymer, that is, yellow discoloration of the polymer, and when the polyester is used in films or hollow bottles, there arises the problem of deterioration in the color tone of the molded articles.
During production of molded articles by melt molding, filter pressure is increased due to clogging of a filter with insoluble particle, to lower productivity.
As an non-antimony catalyst having an excellent catalytic activity and giving polyester free of the problem described above, a germanium compound has been practically used, but this catalyst has a problem that it is very expensive and easily distilled away from the reaction system during polymerization, thus changing the concentration of the catalyst in the reaction system and making control of polymerization difficult, so use of the germanium component as a major catalytic component is problematic.
For preventing thermal degradation of polyester during melt molding, there is also a method of removing a catalyst from polyester. Japanese Patent Application Laid-Open No. 10-251394 discloses a method of removing a catalyst from polyester wherein a polyester resin is brought into contact with an extractant as supercritical fluid in the presence of an acidic substance. However, the method of using such supercritical fluid is technically difficult and leads to higher costs for products, and is thus not preferable.
For the reasons described above, there is demand for a polymerization catalyst which comprises a metal component other than antimony and germanium as a major catalytic component, has an excellent catalytic activity, and gives polyester excellence in (a) thermal stability, (b) thermal oxidation stability and/or (c) hydrolytic stability thus hardly undergoing thermal degradation during melt molding and superior in transparency with less insoluble particle.
The present invention provides a polyester polymerization catalyst which excludes an antimony compound or a germanium compound as a major catalytic component, comprises aluminum as a major metal component, has an excellent catalytic activity, and without inactivating or removing the catalyst, gives polyester not only excellence in thermal stability by effectively inhibiting thermal degradation during melt molding but also superiority in color tone and transparency with less formation of insoluble particle.
Further, the present invention provides polyester which improves thermal stability, formation of insoluble particle and productivity during melt molding thereof into films, hollow molded articles such as bottles, fibers and engineering plastics by using the catalyst, and gives products superior in quality level by using virgin resin thereof or by reutilizing scraps thereof generated during molding, as well as a process for producing polyester by using the polyester polymerization catalyst.
Another object of the present invention is to provide a polyester polymerization catalyst which excludes an antimony compound or a germanium compound as a major catalytic component, comprises aluminum as a major metal component, has an excellent catalytic activity, and without inactivating or removing the catalyst, gives polyester which when formed into melt-molded articles, is not only excellent in heat aging resistance but also superior in color tone and transparency with less formation of insoluble particle.
Further, the present invention provides polyester which improves heat aging resistance, formation of insoluble particle and productivity when melt-molded into films, hollow molded articles such as bottles, fibers and engineering plastics, and so forth by using the catalyst and gives products superiority in quality level by using virgin resin thereof or by reutilizing scraps thereof generated during molding, as well as a process for producing polyester by using the polyester polymerization catalyst.
Another object of the present invention is to provide a polyester polymerization catalyst which excludes an antimony compound or a germanium compound as a major catalytic component, comprises aluminum as a major metal component, has an excellent catalytic activity, and without inactivating or removing the catalyst, gives polyester which when formed into melt-molded articles, is excellent not only in water resistance but also superior in transparency with less formation of insoluble particle.
Further, the present invention provides polyester which improves water resistance, formation of insoluble particle and productivity when melt-molded into films, hollow molded articles such as bottles, fibers and engineering plastics, and so forth by using the catalyst and gives products superiority in quality level in color tone with less discoloration by using virgin resin thereof or by reutilizing scraps thereof generated during molding, as well as a process for producing polyester by using the polyester polymerization catalyst.